The dehydrogenation of alkyl aromatic compounds is conventionally carried out on a commercial scale by passing the alkyl aromatic compound at an elevated temperature through a zone containing a selective dehydrogenation catalyst. Steam is mixed with the alkyl aromatic compound both as a diluent and as a source of heat to raise the alkyl aromatic compound to the dehydrogenation temperature and supply the endothermic heat required for the reaction. Since dehydrogenation also occurs thermally in the absence of a catalyst and this dehydrogenation is non-selective, it is common practice to preheat the alkyl aromatic compound to only about 450.degree. to 500.degree. C., a temperature whereat little, if any, thermal dehydrogenation will occur, and use steam to raise the temperature to the reaction temperature just prior to entry of the admixture into the catalyst bed. Sufficient steam must be added to not only raise the mass to reaction temperature but also to supply the endothermic heat of reaction as the reaction proceeds throughout the bed. The balancing of the steam, both quantity and temperature, to achieve good yields and high selectivities (the percentage of consumed alkyl aromatic compound converted to desired product) has prompted industry to employ two and more often three or more weights of steam per weight of alkyl aromatic compound. Over the years various schemes have been proposed to the end of reducing the steam volume introduced into the reactor. One such scheme which has been implemented on a commercial scale is the heated case reactor as exemplified by Amos et al., U.S. Pat. No. 2,683,180. In this scheme, steam is passed around the catalyst bed in a countercurrent flow to heat the bed and then the steam is mixed with the incoming alkyl aromatic compound. The teaching in the reference patent uses a 3 to 1 steam to alkyl aromatic ratio (S/HC) in the heated case scheme and achieves better results than using the same ratio without heated case, a yield of 86.5 percent versus 79.6 percent. This same reference discloses use of a jacketed reactor heated with flue gas. Similar results were obtained as when the steam was used as compared to unheated case reactors.
Another scheme employed by the industry is the intermediate introduction of steam, best exemplified by Kindler et al., U.S. Pat. No. 3,660,510. This patent discloses 90+ percent yields using three reactors and introducing steam intermediate to provide a 1 to 3 steam ratio in reactor 1; a 2 to 3 steam ratio in reactor 2 and a 3 to 3 steam ratio in reactor 3. This reference illustrates the stepwise conversions when using less than 1 to 1 ratios; i.e., at 1 to 3 conversion is 18 percent, when 2 to 3 is an additional 15-18 and finally at 1 to 1 is an additional 10-12 percent and demonstrates the effect of depending solely on the heat of the steam mixed with the alkyl aromatic to supply the endotherm.
Each of these schemes establishes a process for employing less steam than previous processes or at least obtaining better yields with equivalent steam quantities.
A still further scheme suggested to the industry is that disclosed in Leesemann, U.S. Pat. No. 2,461,331. The patent teaches introducing the alkylaromatic through several distributors located at various levels in the catalyst bed and introducing steam into the bed from the bed surface. The ratio of steam to hydrocarbon is very high throughout a major portion of the bed and generally is 5-40 to 1 even at the exit. Although the reference suggests that a 1 to 1 steam ratio at the outlet can be achieved the early ratios in the bed are higher, i.e., 5-40 to 1 steam to hydrocarbon. Further, the steam is at its highest ratio to hydrocarbon when it is in the greatest ratio. Finally, the process admits of cumbersome equipment and a procedure in which the hottest temperatures are associated with the minimum hydrocarbon.
It is an object of the present invention to provide a scheme which provides an alternative method for achieving improved yields, with low steam ratios. These and other objects will be evident to those skilled in the art to which the invention pertains.